Which Key Date At MS 70?

1847 Gothic Crown. Although I believe they were only struck as proofs.

Do mind if I ask why the coin has to be MS70? It's more likely, IMO, that a classic coin in EF will do better over 20 years than what is being called MS70 today. High end grades, huge price gaps between grades, and eternally changing grading interpretations don't exactly encourage long term holding, it encourages re-submissions.

Twenty years is long time. I don't have much faith that U.S grading is going to carry the load for that long.

I agree with Guido as far as the 1995 W PR70 silver eagle sense there were only 30,125 made. I just looked it up on PCGS's price guide it says 35K. IMHO it can go no were but up.

Sort of depends on how you plan on selling it in the future. Note that if you check out the 2008 Edtion of the Red Book, Pages 399 to 403 you see many coins that sold for hundreds of thousands to millions. These are based on auction prices though. For the average person to buy and sell coins the best thing to do is find coins that the majority of collectors are looking for. Naturally that would include such famous coins as the Lincoln Cents 09S VDB, 22Plain, 55 Double Die. Also, 1856 and 1877 Small Cents, 3 legged 1936D & 37D partial legs, 16D Merc Dime, 32D Washington quarter. There are numerous other coins of great value but you can't beat the ones that have fame and are wanted to complete collections by the masses of collectors. We have 300 million people in the USA alone and probably a large quantity are becoming coin collectors due to all the promotions of the US Mint. Very rare coins may be worth a lot more but just as with a multi million dollar house, you just don't have the amount of buyers available for super rare coins.

The one thing you haven't mentioned is how much money you have to invest. That will have a huge impact on what you CAN or CANNOT buy. What is your budget to work with?